Development of the supersonic missiles is expected to be complete in three or four years, Seoul's Chosun Ilbo daily said.

"Fully-fledged research on the development of supersonic anti-ship missiles has been underway at the Agency for Defense Development (ADD) for years," the newspaper quoted an unidentified South Korean government source as saying.

"The supersonic anti-ship cruise missile is aimed at defending the country against naval threats from other countries in the region rather than North Korea."

Both ADD and Seoul's Defense Acquisition Program Administration declined to comment.

The missile is modelled after Russia's Yakhont cruise missiles and is designed to fly low above water at a maximum speed of 2.5 times the speed of sound, with a range of up to 300 kilometres (186 miles), Chosun said.

Detecting and tracking the missile will be difficult, according to the source.